Fixtures
Configuración de pruebas reutilizable
Fixtures es una lección gratuita de Web3 & DApp Development Fundamentals en CoddyKit. Esta es la lección 4 de 4. Puedes leer la lección completa abajo gratuitamente — luego la practicas en el navegador con un editor de código integrado y un tutor de IA 24/7. Forma parte de la ruta de aprendizaje de Web3 & DApp Development Fundamentals, y tu progreso se sincroniza en la web y la app de CoddyKit. El curso de Web3 & DApp Development Fundamentals incluye 4 lecciones en total.
Partes de esta lección aún no han sido traducidas y se muestran en inglés.
The Setup Problem
Many tests need the same starting state — a deployed contract, funded accounts, initial config. Repeating this setup in every beforeEach is slow and repetitive.
Fixtures solve this by running setup once and snapshotting the result.
What Is loadFixture
Hardhat provides loadFixture in its network helpers. The first time it runs a fixture function and takes an EVM snapshot. Later calls simply revert to that snapshot instead of re-running everything.
This makes tests both fast and perfectly isolated.
Importing the Helper
Import loadFixture from the network helpers package (included in the Toolbox):
const {
loadFixture,
} = require("@nomicfoundation/hardhat-network-helpers");
const { expect } = require("chai");
const { ethers } = require("hardhat");const {
loadFixture,
} = require("@nomicfoundation/hardhat-network-helpers");
const { expect } = require("chai");
const { ethers } = require("hardhat");Writing a Fixture Function
A fixture is an async function that performs setup and returns the objects tests need:
async function deployTokenFixture() {
const [owner, alice] = await ethers.getSigners();
const Token = await ethers.getContractFactory("Token");
const token = await Token.deploy();
return { token, owner, alice };
}async function deployTokenFixture() {
const [owner, alice] = await ethers.getSigners();
const Token = await ethers.getContractFactory("Token");
const token = await Token.deploy();
return { token, owner, alice };
}Using a Fixture in a Test
Call loadFixture at the start of each test and destructure what you need:
it("assigns supply to owner", async function () {
const { token, owner } = await loadFixture(deployTokenFixture);
expect(await token.balanceOf(owner.address))
.to.equal(await token.totalSupply());
});it("assigns supply to owner", async function () {
const { token, owner } = await loadFixture(deployTokenFixture);
expect(await token.balanceOf(owner.address))
.to.equal(await token.totalSupply());
});Why Fixtures Are Fast
Re-running deployment for every test is expensive. With fixtures:
- The setup transactions execute only once.
- Subsequent tests revert the EVM to the snapshot — a near-instant operation.
Large suites can run dramatically faster than equivalent beforeEach setups.
Isolation Guarantee
Because each loadFixture call reverts to the original snapshot, tests cannot leak state into each other. One test transferring tokens does not affect the next test's starting balances.
This isolation is what makes test results reliable and order-independent.
Fixtures with Parameters
loadFixture takes a function reference, not a call, so it cannot accept arguments directly. To vary setup, define multiple fixtures:
async function deployPausedFixture() {
const base = await deployTokenFixture();
await base.token.pause();
return base;
}Each variant gets its own snapshot.
async function deployPausedFixture() {
const base = await deployTokenFixture();
await base.token.pause();
return base;
}Fixtures vs beforeEach
When should you use each?
- loadFixture — preferred for deployment-heavy setup; faster and snapshot-based.
- beforeEach — fine for trivial setup or when you need per-test side effects.
The Hardhat team recommends fixtures for most contract deployment setup.
A Common Pitfall
Do not call the fixture function directly inside loadFixture:
// Wrong: passes the result, not the function
await loadFixture(deployTokenFixture());
// Right: passes the function reference
await loadFixture(deployTokenFixture);Passing the reference lets Hardhat manage the snapshot lifecycle.
// Wrong: passes the result, not the function
await loadFixture(deployTokenFixture());
// Right: passes the function reference
await loadFixture(deployTokenFixture);Sharing Fixtures Across Files
You can move a fixture into its own module and import it wherever needed:
// fixtures.js
module.exports = { deployTokenFixture };
// in a test file
const { deployTokenFixture } = require("./fixtures");
const { token } = await loadFixture(deployTokenFixture);The snapshot caching still works across files in the same run.
// fixtures.js
module.exports = { deployTokenFixture };
// in a test file
const { deployTokenFixture } = require("./fixtures");
const { token } = await loadFixture(deployTokenFixture);Quick Check
Test your understanding of fixtures.
Recap
You learned reusable test setup with fixtures.
loadFixtureruns setup once and snapshots the EVM.- Later calls revert to the snapshot — fast and fully isolated.
- A fixture is an async function returning the objects tests need.
- Pass the function reference, never call it.
- Define multiple fixtures for different starting states.
Preguntas frecuentes
¿La lección «Fixtures» es gratis?
Sí — el texto completo de «Fixtures» es gratis para leer aquí en la web. Para practicarla de forma interactiva (editor de código integrado y tutor de IA 24/7) y desbloquear el resto del curso de Web3 & DApp Development Fundamentals, actualiza a CoddyKit PRO. El curso de Web3 & DApp Development Fundamentals incluye 4 lecciones en total.
¿Qué aprenderé en «Fixtures»?
Configuración de pruebas reutilizable Practicas Web3 & DApp Development Fundamentals con código real que ejecutas directamente en el navegador, y un tutor de IA 24/7 responde tus preguntas mientras trabajas en la lección.
¿Necesito experiencia previa para empezar Web3 & DApp Development Fundamentals?
No se requiere experiencia previa. Web3 & DApp Development Fundamentals en CoddyKit está estructurado para principiantes hasta estudiantes avanzados, así que puedes empezar aquí o desde el inicio y avanzar a tu ritmo. Esta es la lección 4 de 4.
¿Cuánto tiempo toma la lección «Fixtures»?
La mayoría de las lecciones de CoddyKit toman alrededor de 5–10 minutos. Cada una es compacta e interactiva, así que avanzas constantemente y retomas exactamente por donde dejaste en la web y la app.
¿Puedo escribir y ejecutar código en esta lección de Web3 & DApp Development Fundamentals?
Sí. Cada lección de Web3 & DApp Development Fundamentals incluye un editor de código integrado, así que escribes y ejecutas código real directamente en tu navegador y obtienes retroalimentación instantánea de IA — sin configuración local necesaria.